The present invention relates generally to a mixed-flow feedwater heater having a regulating device and more specifically relates to a mixed-flow feedwater heater for a combined gas/steam turbine power plant wherein exhaust gases of a gas turbine are utilized to generate steam for a steam turbine.
In conventional waste heat boilers, it is desirable to maintain a low feedwater temperature at an inlet to the waste heat boiler for the purpose of obtaining the highest possible thermodynamic efficiency. Because of a risk of low-temperature corrosion at the end of the evaporator, however, the temperature is subject to a lower limit. In order to ensure the prevention of a corrosive attack, therefore, the feedwater inlet temperature should be maintained at a level in a range corresponding to a constant, part-load of the boiler. Depending on the specific composition of the gas turbine fuel, the temperature lies approximately between 100.degree. C. and 150.degree. C., and in some instances lower.
The feedwater can be heated in a mixed-flow feedheater either with steam which is bled from the steam turbine or with the heat of steam from a low pressure waste heat boiler located in the exhaust-gas flow after a high pressure waste heat boiler. An evaporator coil of the low pressure waste heat boiler is hereafter termed the feedheater loop.
In the case of feedheating with bleed steam, the steam temperature must be high enough to provide a sufficient range of control during a partial-load operation. This requirement, however, impairs the thermodynamic efficiency.
When feedheating with a feedheater loop, the impairment of efficiency is avoided since exhaust heat which would otherwise be lost is utilized. There then arises, however, the problem of regulating the feedwater inlet temperature to a value which is most favorable for the particular gas/steam turbine power plant operation.
In a known configuration of the combined gas/steam turbine power plant, the regulation of the feedwater inlet temperature is effected by discharging a supply of surplus steam from the mixed-flow feedheater into the condenser of the steam turbine. In this instance, the controlled variable is the steam pressure. The constructional expenditure for such a regulation arrangement, however, is considerable and requires, among other things, various stop valves, pipes and a condenser inlet.
An object of the present invention is to provide a regulating device requiring relatively little constructional expenditure and having a substantially flawless regulation of the feedwater inlet temperature.
The mixed-flow feedwater heater having the regulating device of the present invention includes a feedwater tank having a deaerating dome. Pipes and pumps are included for conveying steam from a low pressure steam drum into the feedheater in order to heat and deaerate the feedwater. The pipes and pumps permit the feedwater from the mixed-flow feedheater to be conveyed into the low pressure steam drum or permit the feedwater to be conveyed through the low pressure evaporator. The water level in the low pressure steam drum is regulated to a desired level. The feedwater tank is connected to the low pressure steam drum by a steam pipe having a pressure-regulating valve subjected to the pressure in the feedwater tank. The valve limits the pressure, and hence the saturated steam temperature, in the mixed-flow feedwater heater to a specified, predetermined, adjustable value.
In another form of the invention feedwater pumps are provided for both the low pressure steam drum and for the high pressure steam generator.
Still another form of the present invention has only a high pressure feedwater pump which also supplies the low pressure steam drum with feedwater by way of a branch pipe.